1. Field of the Invention
The present invention relates to data communications. More specifically, the present invention relates to differentially encoded and decoded communications systems that communicate information across a plurality of sub-carriers within intermittently transmitted blocks of information.
2. Description of the Related Art
Modern communications systems have advantageously employed the use of multiple sub-carriers as a means of utilizing available bandwidth. Two examples of such use include orthogonal frequency division multiplexed systems (“OFDM”) and multiple-carrier transmission systems (“MCT”). In multiple-carrier transmission and OFDM systems, a given portion of bandwidth is sub-divided into several sub-bands and information is modulated onto each of the sub-bands. MCT systems are particularly suitable for the implementation of a frequency division multiplexed multiple access (“FDMA”) systems.
FDMA system protocols service several individual users over one frequency band by devoting particular frequency slots to specific users or uses. This is by frequency division multiplexing the information associated with different users or uses. Knowledge of the frequency slot in which any specific information resides permits reconstruction of each user's information at the receiving end of the communication channel. This multiplexing is independent of the information encoding, or modulation, scheme employed. It is also known in the art to time division multiplex and provide multiple user access (“TDMA”) within the aforementioned FDMA structure.
OFDM systems address a problem that is faced, for example, when pulsed or intermittent signals are transmitted in FDMA, and certain other formats. In accordance with principles well known in the communication sciences, the limited time duration of such signals inherently broadens the bandwidth of the signal in frequency space. Accordingly, adjacent frequency sub-channels may significantly overlap, defeating the use of frequency as a user or purpose identifying parameter. However, pulsed information that is transmitted on specific frequencies can be separated, in accordance with OFDM principles, despite the fact that the frequency channels overlap due to the limited time duration of the signals. OFDM generally requires a specific relationship between the data rate and the carrier frequencies. Specifically, the total signal frequency band is divided into N frequency sub-channels, each of which has the same data rate 1/T. These data streams are then multiplexed onto a plurality of carriers that are separated in frequency by 1/T. Multiplexing signals under these constraints results in each carrier having a frequency response that has response zeroes at multiples of 1/T. Therefore, there is no interference between the various sub-carrier channels, despite the fact that the channels overlap each other due to the broadening associated with the data rate and intermittent transmission.
Parallel data transmission is a technique related to FDMA. It is also referred to as multi-carrier transmission (MCT), as mentioned above. MCT has significant calculational advantages over simple FDMA. In this technique, each user's information is divided and transmitted over different frequencies rather than over a single frequency, as in standard FDMA. In an example of this technique, input data at NF bits per second are grouped into blocks of N bits at a data rate of F bits per second. N carriers are then used to transmit these bits, each carrier transmitting F bits per second. The carriers can be spaced in accordance with the principles of OFDM. Respecting MCT systems and the use of pulsed or intermittent signals, the same spectral spreading discussed above does occur. Designers employ various techniques to minimize this problem. Channel matching filters are frequently employed to control spectral spreading. Such filters can be implemented with fast Fourier transforms (“FFT's”), with certain finite impulse response filters, such as Nyquist filters, and through the implementation of other techniques as are understood by those skilled in the art. There are other approaches to minimizing this problem, including selection of sub-channelization and guard-band widths to maintain adequate margin in the channel spacing plan.
Both the phase and the amplitude of the carrier can be varied to represent the information in MCT (and OFDM) systems. Accordingly, multi-carrier transmission can be implemented with M-ary digital modulation schemes. In an M-ary modulation scheme, two or more bits are grouped together to form symbols and one of the M possible signals is transmitted during each symbol period. Examples of M-ary digital modulation schemes include Phase Shift Keying (PSK), Frequency Shift Keying (FSK), and higher order Quadrature Amplitude Modulation (QAM). In QAM a signal is represented by the phase and amplitude of a carrier wave. In high order QAM, a multitude of points can be distinguished on an amplitude/phase plot. For example, in 16-ary QAM, sixteen such points can be distinguished. Since four bits of zeros and ones can take on sixteen different combinations, a four-bit sequence of data symbols can, for example, be modulated onto a carrier in 16-ary QAM by transmitting only one value set of phase and amplitude, out of the possible sixteen such sets.
In both multi-carrier transmission and OFDM systems, the encoding and decoding can occur either coherently or differentially. Coherent detection techniques generally require that the channel be somehow estimated to obtain an absolute reference to phase and/or amplitude for each sub-carrier. In contrast, differential detection does not perform any channel estimation, thereby reducing complexity and eliminating the need for any pilot tones or other reference encoding techniques. Rather, differential detection compares each transmitted carrier state with another transmitted carrier state to establish the change in phase or amplitude between the two, which is defined as a symbol of information. Information is encoded as the difference in phase and/or amplitude between carrier states, and, decoding is accomplished by detecting the difference in phase and/or amplitude between carrier states.
Differential detection is affected by various noise sources, including Doppler shift, multi-path distortion, Rayleigh fading, and other noise. The ability to decode information is greatly affected by such noise sources, and it is therefore desirable to reduce the effects caused by such noise. As the symbol decoding in the differential decoding process are taken from positions further apart in either time or frequency, the affects of noise increase. The originally transmitted information is correlated in time and frequency as it is generated, and the effects of noise tend to decorrelate the information. Therefore, designers of differential detection system almost universally specify that differential detection occur between adjacent carrier states, adjacency being defined in terms of either time or frequency. This technique minimizes the de-correlation effects of noise. Since the information is encoded as the difference between carrier states, the first transmitted carrier state contains no decodable information in and of itself. The second through the last carrier states do contain information when compared to the previous, adjacent, carrier state. In a continuously transmitting system, the fact that the first transmitted carrier state contains no information does not represent the loss of any significant amount of information.
In the case of pulse or intermittent transmission systems, the fact that differential encoding and decoding requires two carrier states for the transmission of each symbol can result in a loss of information, and therefore a loss in system performance, which is typically measured as a reduced signal to noise ratio. A typical prior art differential encoding/decoding system that employs multiple sub-carriers to transmit intermittent signals can operate in one of two ways. In the first case, N−1 symbols are encoded across N frequencies by transmitting a group of N sub-carriers simultaneously. A first sub-carrier state at a first frequency is used as the reference to an adjacent frequency sub-carrier state, the differential between the two encoding a single symbol. The next adjacent sub-carrier is a compared to the previous and so forth until the sequential difference between all of the transmitted sub-carriers is assessed to decode the N−1 symbols of information. In the second scenario, N−1 symbols are encoded on each sub-carrier across N intervals in time. Each sub-carrier state at the first interval in time is compared to the same sub-carrier's state at the second interval in time. The sequence proceeds through all N intervals.
The arrangement of a certain number of sub-carriers over a certain number of symbol intervals, that is used to transmit each interval block of data, can be conceptualized as an array (which dimensions are the number of sub-carriers by the number of symbol intervals) for encoding and decoding the transmitted symbols. Since the number of symbols encoded is N−1 for either the number of sub-carriers or the number of symbol intervals, there is always a corresponding reduction in performance associated with such systems.
Available bandwidth is always constrained in a communication system design. Either by limited allocation of radio spectrum in the case of radio transmission systems, or in limited transmission capabilities of wireline systems, which include inductive, resistive, and capacitive losses. Time is usually constrained as well, either by the need for systems to emulate real-time responsiveness, or by the requirement to maximize the data throughput of the system. Thus, there is a need in the art to improve the performance of differentially encoded data communication systems which operate on a plurality of sub-carrier by transmitting intermittent, or pulsed, information blocks.